The present invention relates to adducts for use in antiperspirants, and processes for making and using such adducts.
Various antiperspirant formulations are well known in the cosmetic art. Certain ingredients of the formulation will always be present, while others will depend upon the particular form of the antiperspirant, e.g., a stick, a gel, a lotion, or an aerosol. For example, sticks often contain an organic liquid carrier, a gelling agent that provides the antiperspirant stick with its solid character, and an active antiperspirant ingredient. Antiperspirants are applied to an area of the body such as the axilla by rubbing to deposit a layer of antiperspirant. Accordingly, it is desirable that the ingredients used in any antiperspirant formulation result in an antiperspirant which is smooth, non-greasy and non-tacky.
Certain disadvantages exist with current formulations including the fact that a white chalky residue is often left on the body and transferred to clothing, and that the formulation can break down after storage, e.g., a stick can shrink or become flaky and crumbly. A chalky residue after use of a stick antiperspirant is due in part to the fact that relatively large particles of the antiperspirant salt are employed in stick antiperspirants. Since the original stick itself is white, the deposit on the skin is also white.
Accordingly, it would be desirable to produce antiperspirants which are transparent or translucent as well as having the other desirable properties for an antiperspirant. One factor which determines the optical appearance of a dispersion formulation is the particle size of any ingredient which is present in solid form.
The question of the effect of particle size upon optical appearance has been investigated by the paint industry. To develop hiding power, a pigment must be subdivided until the refractions, diffractions, and reflections produced by the many tiny particles are capable of reversing the direction of light rays which strike the particles. The smaller the particle diameter, the greater the number of individual particles in any specified weight or volume, and consequently, the greater the number of pigment interfaces to interfere with the linear transmission of light. Thus, it might be expected that a pigment manufacturer would strive for the smallest possible particle size to obtain the greatest possible hiding power. However, it is also known that a particle disappears optically when its size has been sufficiently reduced.
Studies have shown that the ability of a particle of any given material to scatter or to diffuse light of a particular wavelength is a function of its particle size relative to that wavelength. Various estimates have placed the most effective particle diameter for hiding power at approximately one-half the wavelength of the light involved. Therefore, as the diameter of a pigment particle becomes increasingly smaller than one-half of the shortest wavelength of visible light, about 4,000 angstroms for violet, it begins to disappear because it loses its ability to produce visible interference with the passage of light waves.
However, in the case of particles to be used in antiperspirant products, additional considerations must be taken into account when estimating the optimum antiperspirant particle size needed to achieve clarity. Particle size and particle population or concentration must be carefully balanced. Since antiperspirant compositions require a very high concentration of active ingredients as compared to the model systems described in the colloid scientific literature, the particle sizes to obtain optical clarity must not exceed about 0.20 micron and preferably 0.10 micron.
Unfortunately, when such small particle sizes are used, other factors become important, such as how to prevent such small particles from agglomerating to reform larger particles that could no longer be suited for clear colloidal dispersions. While this area has been the subject of a sizable amount of research, little success has been achieved and only very few substances have been successfully made into stable colloidal dispersions. For example, Markovic et al, "Structural and Dynamic Features of Concentrated Non-Aqueous Dispersions", Colloids and Surfaces, 24 (1987), 69-82, and "Small Angle Neutron Scattering Studies on Non-Aqueous Dispersions of Calcium Carbonate", Colloid and Polymer Science, 262 (1984), 648-656, describe colloidal dispersions of calcium carbonate in toluene. The core particles of calcium carbonate are stabilized by an adsorbed layer of an alkyl aryl sulfonic acid.
Another study of this effect is an article by Mates et al, "Steric Stability of Alkoxy-Precipitated TiO.sub.2 in Alcohol Solutions", Colloids and Surfaces, 24 (1987), 299-313. That particular study was an experimental program which evaluated the suitability of various surfactants as steric stabilizers for ethanol suspensions of alkoxy-precipitated TiO.sub.2 particles. The purpose of the study was to determine the most suitable surfactants for preventing agglomeration and/or aggregation of the TiO.sub.2 particles.
Before proceeding further, a definition of terms used in fine particle size technology is in order. A particle is defined as a single unit of material which can be clearly discerned in a fine particle system either by direct observation or by light or electron microscopes.
Individual particles may be associated into agglomerates or aggregates. Particles in an agglomerate are only loosely associated while in an aggregate, the particles are held together strongly to form a ball or block that acts as a distinct particle for all practical purposes. Therefore, an agglomerate is a loose confederation of particles that can change when it is handled, whereas an aggregate is a strongly welded assembly of particles that will maintain its group identity under normal handling conditions.
A large number of particles, either agglomerated or aggregated or both, is said to constitute a powder. Generally a powder is regarded as being constituted of particles in the size range of approximately 0.1 to 1,000 microns.
Particles less than 0.1 micron are normally dispersed in a vehicle and regarded as constituting a colloidal dispersion. If the vehicle is organic, the dispersion is referred to as an organosol. In the dispersions covered by this invention, stability with the total absence of flocs is mandatory. Flocs are clusters of low-strength agglomerates and aggregates loosely attached to each other by Van der Waals forces.
The behavior of a powder system is determined by surface, inertial, gravitational, and other macroscopic forces. In a colloid, surface energy, capillary attraction, and surface charges very frequently dominate the behavior of the system.
Also, binding forces between particles start assuming considerable proportions as particle size decreases. This is an extremely important consideration when one wishes to attain optical clarity by means of particle size reduction because one must divide the antiperspirant salt to ultrafine size of 0.2 and preferably 0.1 micron or less, suspend the particles in a vehicle, and keep the particles from regrouping to form agglomerates, aggregates, and flocs despite the extremely high particle-to-particle adhesion energies that develop. Furthermore, an ultrafine particle formed by any technique must never be allowed to combine with a sister particle to form the various clusters as previously cited. Thus a particle must be stabilized immediately as it is formed by surrounding it with a protective layer or shell to shield it from contact with another particle during collision. Thus, one must not only reduce the particles to an appropriate size, but must also stabilize those particles.
Finally, two types of colloidal systems exist, viz: sols and gels. A sol consists of discrete, separate particles (normally solids) dispersed in a continuous phase (normally, though not necessarily, a liquid) and resembles a solution in many respects. A gel, however, comprises two continuous phases with one of them normally being a solid.
Accordingly, a need exists for a sol (organosol) of an antiperspirant salt in a cosmetic organic liquid (e.g., an emollient or moisturizer) which is stable during storage even though it is made up of ultrafine particles. A need also exists for an antiperspirant salt in particle sizes sufficiently small to be below the wavelength of light, i.e., substantially below 0.4 microns, thereby resulting in transparency at high concentration. Moreover, a need exists for a method for preparing such particles such that the particle size distribution initially produced, and the distribution maintained over a period of time, are sufficiently uniform that the organosol retains its original transparency or translucency.